Dispersed-phase structure in sheared MHD turbulence

Abstract

Direct numerical simulations are performed to examine particle dispersion in homogeneous turbulence subjected to mean shear and an externally applied magnetic field. We employ the Brucker et al. algorithm for simulating mean shear without the remeshing required in the traditional Rogallo scheme. We demonstrate that the Rogallo approach requires higher grid resolution to match the Brucker et al. approach at high shear rates. Both the shear and the magnetic field are uniform. The applied magnetic field is aligned with the direction normal to the plane of the mean shear. Results are presented for three values of the ratio, M, of the mean shear time scale τ shearto the Joule time scale τ m. The dispersed particles all have response times near unity when scaled on the Kolmogorov time scale at the instant of magnetic field application. We find that the dispersed-phase structural anisotropy deviates noticeably from the anisotropy of the velocity field structure. The dispersed-phase anisotropy is determined by M and by the magnetic Reynolds number Re m. When M is small, the mean shear dominates and the particle distribution appears isotropic even though the turbulent eddies align preferentially along the streamwise direction. When τ shearis much larger than τ m, sheets of particles align along both streamwise and spanwise directions. Stronger particle clustering in the streamwise direction is observed when Re m= 20, indicating that the magnetic effect on dispersed-phase structure is less significant at moderately higher magnetic Reynolds numbers. When τ shearis comparable to τ m, the particles disperse broadly across the flow at Re m= 1 because the mean shear and the magnetic field are equally effective in inducing dispersed-phase anisotropy. Particle clustering along the mean streamwise direction is obtained when Re m= 20 due to weaker influence of the magnetic field. © 2012 Taylor & Francis.